Most head/eye tracking systems employ illumination of the object (i.e. the eyes of a user) in order to improve performance of the system. In order for such illumination to be distinguishable, the system should, as far as possible, be able to suppress ambient light.
In order to eliminate or minimize interference from ambient light, the illumination of a head/eye tracking system can be restricted to a narrow wavelength range, preferably outside the visible light spectrum. Typically, a light source having a light emission spectrum concentrated around a distinct center wavelength outside the visible spectrum is used in combination with a band-pass filter having a pass-band centered around the center wavelength.
The light source can be a solid state light source, such as a LED with a center wavelength in the near infra red (NIR) region, e.g. 840 nm or 940 nm. The filter has a pass-band enabling capturing of most light emitted by the light source, while at the same time blocking most ambient light. As an example, the pass-band of the filter is ±25 nm.
One particular challenge is sunlight, which has a relatively high irradiance (flux per area) over a broad spectrum, including NIR. In particular some automotive applications present challenges with strong sunlight.
In the NIR region, the spectral irradiance (flux per area per wavelength) of sunlight is approximately 1 W/m2/nm, so that the total irradiance of sunlight admitted by the filter mentioned above is around 50 W/m2. In order to distinguish illumination from the light source from sunlight, the irradiance of the light source (flux per area) thus needs to be in the same order of magnitude (50 W/m2). The required electrical power can be limited by using pulsed light, and in a typical implementation the light source has an electrical power as low as 1-2 W.
One issue in this context is that light sources such as LEDs have a temperature drift, i.e. the center wavelength will shift slightly when the temperature changes. As an example, for a typical LED the temperature drift is a few tenth nm/K. The pass-band of the filter therefore needs to be chosen to correspond to the LED emission spectrum in the expected operating temperature. For many applications, e.g. indoor applications without significant temperature variations, this will not present any significant problem. However, in some applications, such as automotive applications, there will be significant variations in the operating temperature of the LED.
In a typical automotive installation, the LED temperature may change from an initial temperature, which may be as low as minus 30 degrees Celsius or less, up to a steady-state operating temperature of the LED circuitry. Depending on the ambient temperature and installation-specific thermal resistance, this steady-state operating temperature may be as low as 20 degrees Celsius and as high as 90 degrees Celsius. This corresponds to a shift of the LED center wavelength of around 30-40 nm, i.e. in the same order of magnitude as the pass-band of the filter. In worst case, most of the emitted light will be lost. If also the initial phase is accounted for, i.e. the time period before the LED reaches the steady state operating temperature, the active operating temperature range becomes even greater.
In order to compensate for this temperature drift, and ensure satisfactory system performance also in a worst-case scenario, the pass-band of the filter may be wider, so as to ensure that most of the power emitted by the LED will pass the filter at all expected temperatures. However, as a consequence, the filter will also allow more sunlight to pass, and therefore the power of the LED needs to be increased, typically by a factor of two or three.